A field-emission cathode (or field emitter) contains a group of electron-emissive elements that emit electrons upon being subjected to an electric field of sufficient strength. The electron-emissive elements are typically situated over a patterned layer of emitter electrodes. In a gated field emitter, a patterned gate layer typically overlies the patterned emitter layer at the locations of the electron-emissive elements. Each electron-emissive element is exposed through an opening in the gate layer. When a suitable voltage is applied between a selected portion of the gate layer and a selected portion of the emitter layer, the gate layer extracts electrons from the electron-emissive elements at the intersection of the two selected portions.
The electron-emissive elements are often shaped as cones. Referring to the drawings, FIGS. 1a-1d illustrate a conventional technique as, for example, disclosed in Spindt et al, U.S. Pat. No. 3,755,704, for creating conical electron-emissive elements in a gated field emitter for a flat-panel CRT display. At the stage shown in FIG. 1a, the partially finished field emitter consists of an electrically insulating substrate 20, an emitter electrode layer 22, an intermediate dielectric layer 24, and a gate layer 26. Gate openings 28 extend through gate layer 26. Corresponding, somewhat wider dielectric openings 30 extend through dielectric layer 24.
Using a grazing-angle deposition procedure, a lift-off layer 32 is formed on top of gate layer 26 as depicted in FIG. 1b. Emitter material is deposited on top of the structure and into dielectric openings 30 in such a way that the apertures through which the emitter material enters openings 30 progressively close. In U.S. Pat. No. 3,755,704, a closure material is simultaneously deposited at a grazing angle to help close the deposition apertures. Generally conical electron-emissive elements 34A are thereby formed in composite openings 28/30 over emitter layer 22. See FIG. 1c. A continuous layer 34B of the emitter/closure material forms on top of gate layer 26. Lift-off layer 32 is subsequently removed to lift off excess emitter/closure-material layer 34B. FIG. 1d shows the resultant structure.
Utilization of lift-off layer 32 to remove excess emitter/closure-material layer 34B is disadvantageous for various reasons. Portions of the lift-off material invariably accumulate along the side edges of gate layer 26. This reduces the size of the openings through which the emitter material is initially deposited and makes it difficult to scale down electron-emissive elements 34A. The grazing-angle deposition of lift-off layer 32 becomes increasingly difficult as the lateral area of the field emitter increases and thus presents an impediment to scaling up the field-emitter area.
The lift-off material deposition must be performed carefully to assure that no lift-off material accumulates on emitter layer 22 and causes cones 34A to be lifted off during the lift-off of excess layer 34B. Since layer 34B is removed as an artifact of removing lift-off layer 32, particles of the removed emitter material can contaminate the field emitter. Furthermore, deposition of the lift-off material takes fabrication time and therefore money.
Wilshaw, PCT Patent Publication WO 96/06443, discloses a process for manufacturing a gated field emitter in which each electron-emissive element consists of a molybdenum cone situated on a cylinder. The electron-emissive elements are formed over a bottom metal layer. Using an aqueous electrolytic solution, Wilshaw applies a potential of 2-4 volts to a niobium gate layer in order to electrochemically remove a layer of excess molybdenum that accumulated over the gate layer during the deposition of molybdenum through openings in the gate layer to form the conical portions of the electron-emissive elements.
Just before electrochemically removing the excess molybdenum, Wilshaw removes the bottom metal layer. Consequently, Wilshaw's electron-emissive elements are electrically isolated from one another during the electrochemical removal of the excess emitter material. Inasmuch as some electron-emissive elements may be electrically shorted to the excess molybdenum during the electrochemical removal step, Wilshaw needs this isolation to protect the unshorted electron-emissive elements since they could otherwise be electrically shorted through the back metal layer and the shorted elements to the excess molybdenum and thus could be electrochemically attacked in removing the excess molybdenum. Later, Wilshaw performs an operation on the back surface to nullify the presence of shorted electron-emissive elements. Finally, Wilshaw forms a resistive layer over the bottoms of the electron-emissive elements, and a layer of emitter electrodes over the resistive layer.
Wilshaw's electrochemical removal technique avoids the necessity to use to use a lift-off layer for removing the layer of excess emitter material. However, removing the back metal layer before electrochemically removing the excess molybdenum and then creating emitter electrodes after completing the electrochemical removal is time-consuming and requires several complex processing steps. Performing the additional electrical short nullification operation further increases the fabrication time and complexity. In fabricating a gated field emitter having electron-emissive elements at least partially shaped as cones, it is desirable to have a technique for removing a layer that contains excess emitter material without incurring the fabrication inefficiency of Wilshaw or the fabrication difficulty involved in utilizing a lift-off layer.
Wilshaw's use of an aqueous electrolytic solution for removing the excess molybdenum poses difficulties. The high charge-to-radius values of the ions of metals, such as molybdenum (whose normal ionic charge state is plus six), cause these metals to precipitate readily out of aqueous electrolytic solutions as metal hydroxides, metal oxides, and/or hydrated metal oxides. The precipitates coat the electron-emissive elements and destroy their usefulness. Wilshaw's electrochemical removal potential of 2-4 volts which presumptively overcomes the precipitation problem without causing electrochemical removal of the niobium in the gate layer is quite high and could result in significant electrochemical attack of many other highly attractive candidates for the gate metal. It is desirable to have an easier, more flexible way to avoid unwanted precipitation.